1.Field of the Invention
The invention relates to an optical waveguide induced by electro-optic working, and to devices comprising such a waveguide. Interaction between light and an applied electric field causes in a medium, at least if an electro-optic coefficient of it has a certain magnitude, such a refraction index distribution that a light wave guiding path in this medium is defined.
2. State of the Art
An optical waveguide of the type described above is known from for example the reference (1) mentioned under D. According to this known technique a single-crystal substrate of KNbO.sub.3 in the form of a plate cut perpendicular to the b-axis, is used. The upper surface of this plate is provided with two parallel strip-shaped electrodes disposed adjacent to each other. An electrostatic field, the principal direction of which coincides with that of the c-axis, can be effected by means of these electrodes. Interaction between this field and light causes an electro-optic effect in the substrate. More in particular a TE-mode light wave guiding path is induced in this substrate, at least in so far as the field vector, the optical axis of the material of the substrate and the electric vector of the light mainly follow one and the same direction.
Such a known structure has the following drawbacks:
small degree of freedom with respect to the possibility of using and the way of application; PA0 the density of the electro-static field and consequently the light-field interaction is relatively strongly dependent on a certain position; PA0 one light wave guiding path requires two parallel electrodes disposed adjacent to each other in one and the same plane; PA0 a combination crystal plate-electrodes is a "bulk"-structure which lends itself less favourably to be used in the field of integrated optics. PA0 a substantially flat portion of electro-optic material; PA0 a first electrode formed in a preselected pattern on one major surface of said portion; PA0 a second electrode formed on an opposing surface of said portion; PA0 a buffer layer interposed between said first electrode and said portion of electro-optic material; and PA0 means for causing a voltage to occur across said portion of electro-optic material through said first and second electrodes. PA0 parasitic coupling between light wave guiding paths is eliminated; PA0 the attenuation introduced during light wave transmission is relatively low; PA0 simplified manufacturing process; PA0 increased number of separate light wave transmission paths per unit of surface area is readily implemented; and PA0 feasibility to induce light wave guiding paths having substantially equivalent transmission characteristics.
The reference (2) mentioned under D discloses several sorts of integrated optical devices, such as a modulator and a switch, the working of which is also based on the electro-optic effect. The basic structure proposed in the scope of this known technique is in essence in conformity with what is disclosed in the above-mentioned reference D (1). Reference D (2) adds to this an embodiment of an electro-optic switch with three parallel strip electrodes disposed in one and the same plane. Such an embodiment has in principle the same drawbacks as those mentioned in connection with reference D (1). Moreover, it is required that the central electrode has a very small width compared with the side electrodes, which is a drawback from a manufacture-technical point of view.
The reference (3) mentioned under D discloses electro-optically induced strip waveguides, use being made of a planar LiNbO.sub.3 substrate containing a diffused thin Ti-film. In this connection the relations between the division of the optical field and possible propagating modes are considered in this reference. Attention is called to the possibility to utilize the effect, that there can be various pq-modes due to an external electric field, for modulating the intensity of an optical signal. This known technique too has the same drawbacks as those mentioned in connection with reference D(1).
The reference numbered (6) under D describes a light waveguide device comprising:
These prior art devices in general comprise a transparent body of electro-optic material, more in particular a crystal plate of sizable thickness. Such a body or plate of electro-optic material by itself provides a substrate of an optical integrated circuit to be provided. Therefore the composite device including such body or plate and first and second electrodes arranged on opposing major surfaces of such a body or plate, actually is a "bulk"-structure. Such bulk-structures inherently fail to meet requirements of modern technologies in the field of manufacturing integrated optical circuits. Moreover due to the sizable thickness of a crystal plate capable of exhibiting the electro-optic effect the density of the electrostatic field induced by an electric voltage applied across such crystal plate, to a certain extent is non-uniform. Further the overall voltage required to induce a light wave guiding path therein is relatively high, which for many applications is highly adverse. In addition this prior art is typically intended for multimode light. In view of these facts the crystal body portion lying beneath the electrode pattern does not form a sharply and well defined light transmission path therein. Therefore the degree of freedom to use and apply the above prior art is detrementally restricted.